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Minimal CISC
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The minimal CISC instruction set presented here is stack-based and composed entirely of zero-address instructions. There are only 8 instructions, so each can be coded as a 3-bit syllable. Assuming a 16 bit word, 5 instruction syllables can be packed in each word (the least significant syllable is the first). This writeup is a revision of a paper by the same name published in ACM Computer Architecture News, 16, 3 (June 1988), pages 56-63.
The instructions are:
NOP: No operation. DUP: Duplicate the stack top. This is the only way to allocate stack space. ONE: Shift the stack top left one bit, shifting one into the least significant bit. ZERO: Shift the stack top left one bit, shifting zero into the least significant bit. LOAD: Use the value on the stack top as a memory address; replace it with the contents of the referenced location. POP: Store the value from the top of the stack in the memory location referenced by the second word on the stack; pop both. SUB: Subtract the top value on the stack from the value below it, pop both and push the result. JPOS: If the word below the stack top is positive, jump to the word pointed to by the stack top. In any case, pop both.